I just acquired a Pentax 6x7 MLU. It was given by a friend. The camera was stored and was not used for decades and acquired some dirt. Anyway, getting straight to the point, the camera won't work!
I installed a battery (which was stored in my room for a long period of time but I remember it wasn't used) and tried to battery check, there's no any light appearing. The battery bay is all clean, no corrosion. I loaded the camera with testing film, wind it, pressed the shutter button, but the mirror will just stucks half way and won't slap fully. In order for the mirror goes up all the way, I have to press the Shutter/Mirror Reset Button, then the mirror goes to Mirror Luck Up position, and will finally release when pressing the Shutter Button again. And I suspect that the Shutter Curtain does not open and close during that time.
This is the first time I use a Pentax 6x7, I read a downloaded its user manual and watched some Youtube videos on how to operate it, but the camera won't work. Anyone can help me with this?
Greatest thanks!

I can't say I recognize any of the symptoms. But I would buy a fresh, new battery, remove the prism ( I assume it has one) and clean all contact points common to the prism/body interface and the battery holder before starting to trouble shoot the camera. Clean the metal contacts with an eraser or something.

I would buy a new batterie too. The one with lithium have always been the best when it come to power bodies.

To me, this sounds like the electrical power might not be enought to go trough the whole process because the mechanics are maybe a bit sticky inside.

You have to know that the 6x7 body don't work without film inside, or at least some looooong roll of paper so that the whole mechanisme is armed when you "reload the frame". So my question is, do you have film inside ?

I installed a battery (which was stored in my room for a long period of time but I remember it wasn't used) and tried to battery check, there's no any light appearing.

The P6x7 and 67 need a working battery for the shutter to work. If pressing the battery check button and you don't get a red light the do not use the camera. My P67 had a dead battery and the "New" one I installed also did not work as it had been sitting in my camera bag for a couple years. Had to buy a new one, get a Silver Oxide if you can, as the are more reliable that alkaline.

While most respondents are pointing the finger at a battery, to me this sounds more like a lazy mirror actuating solenoid, something that is age-related. The 6x7 and 67 cameras use archaic technology that is prone to "lose it" over time: shutter speed resistors, shutter/mirror solenoids, TTL meter sensitivity... all manner of things that your usual 1970s camera might never even dream of having, but the 6x7 and 67 cameras, if there is a risk of it, it will happen! A similar problem can happen to the shutter, either opening partially or not closing fully. You said it has not been used in decades, and that provides a valuable clue. A camera like the 6x7 needs work, and lots of it. A camera that has been mothballed for an extended time must be expected to give trouble. I recall something in the Pentax 6x7 engineering manual that it is true also that having the shutter cocked for an extended period of time (months and months) will effect shutter accuracy. There are all manner of idiosyncracies with the 6x7 / 67 cameras and wising up to them and paying attention to workarounds/prevention can work wonders. As per Phil's last line, above, I also use silver oxide batteries in my 67.

You make me think that i should do a "blank roll" without film in it, just to fire it at all speed many times. I have sometime some misfire at some speed (like 1/1000 or 1/125). maybe using them more will "clean" a bit the mecanisme from old grease ?